1. Field of the Invention
The present invention relates to laser communications and more particularly to ways for amplifying signals in laser communications.
2. Brief Description of Prior Developments
The receiver sensitivity for an atmospheric laser communications system is increased by focusing the free-space light into a multimode fiber amplifier that pre-amplifies the received signal. For laser communications in the 1500-1600 nm region, we have used a multimode erbium-doped fiber amplifier (EDFA). Other fiber amplifiers are applicable for other wavelengths. Typical systems use readily available single mode EDFAs as receiver preamps, but this arrangement typically results in a 5-10 dB coupling loss of signal into a single mode EDFA from free-space, even with the benefit of adaptive optics correction. Multimode fibers have a larger core diameter and a larger numerical aperture (NA), thus permitting the collection of aberrated free-space optical wave fronts with minimal losses. Amplifying this signal with a multimode EDFA provides increased detector sensitivity compared to a (multimode) detector with no preamp, and also better net receiver sensitivity than a single mode preamp, which has an inherently high coupling loss.
An improved method and apparatus for amplifying signals in laser communications.